1. Field of the Invention
The present invention relates to a displacement pickup which picks up a displacement of a scale through the use of an optical interference.
This application claims the priority of the Japanese Patent Application No. 2002-308931 filed on Oct. 23, 2002, the entirety of which is incorporated by reference herein.
2. Description of the Related Art
Conventionally, a grating interferometer is used to pick up a displacement of gratings recorded on a moving scale through the use of an optical interference. A conventional displacement pickup will be described below with reference to FIG. 1. The displacement pickup is generally indicated with a reference number 4. It should be noted that the displacement pickup 4 in FIG. 1 uses a transmission grating.
As shown in FIG. 1, the displacement pickup 4 includes a coherent light source 90, first lens 91, first polarization beam splitter (PBS) 92, first quarter-wavelength plate 93, reflection prism 94, second quarter-wavelength plate 95, second lens 96, beam splitter (BS) 97, second PBS 98, first photoelectric transducer 99, second photoelectric transducer 100, third quarter-wavelength plate 101, third PBS 102, third photoelectric transducer 103, fourth photoelectric transducer 104, first differential amplifier 105, second differential amplifier 106 and an incremental signal generator 107. The displacement pickup 4 constructed as above reads transmission gratings recorded on a scale 108.
The coherent light source 90 emits a beam of light to the first lens 91. The first lens 91 converges the incident light beam into an appropriate light beam and lets the light beam travel to the first PBS 92. The first PBS 92 splits the incident light beam into two light components: an S-polarized light component and a P-polarized light component. It should be noted that the first PBS 92 reflects the light beam containing the P-polarized light component and allows the light beam containing the S-polarized light component to pass through. Also note that if the beam of light emitted from the coherent light source 90 is a plane-polarized light, it has the polarized direction thereof turned 45 deg. for incidence upon the first PBS 92. Thus, it is possible to equalize the intensity of the light beam containing the S-polarized light component with that of the light beam containing the P-polarized light component.
The light beam containing the S-polarized light component is incident upon a P point on the gratings recorded on the scale 108, while the light beam containing the P-polarized light component is incident upon a Q point on the gratings. They are diffracted in directions, respectively, given by the following formula:sin θ1+sin θ2=n·λ/Λwhere θ1 indicates an angle of incidence upon the scale 108, θ2 indicates an angle of diffraction from the scale 108, Λ indicates a pitch (width) of the gratings, λ indicates the wavelength of the light beam, and n indicates a diffraction order.
In the displacement pickup 4, such an adjustment is made that on the assumption that the angle of incidence upon the P point is θ1p, the angle of diffraction from the P point is θ2p, angle of incidence upon the Q point is θ1q and angle of diffraction from the Q point is θ2q, θ1p=θ2p=θ1q and θ2q. Also, the diffraction order is the same at the P and Q points.
The light beam diffracted at the P point passes through the first quarter-wavelength plate 93, is reflected vertically by the reflection prism 94 back to the P point, and is diffracted by the grating. At this time, the light beam having returned to the P point is a P-polarized light component since the optical axis of the first quarter-wavelength plate 93 is inclined 45 deg. in relation to the polarized direction of the incident light beam.
Similarly, the light beam diffracted at the Q point passes through the second quarter-wavelength plate 95, is reflected vertically by the reflection prism 94 back to the Q point, and is diffracted by the grating. At this time, the light beam having returned to the Q point is an S-polarized light component since the optical axis of the second quarter-wavelength plate 95 is inclined 45 deg. in relation to the polarized direction of the incident light beam.
The light beams having been diffracted again at of the P and Q points return to the first PBS 92. Since the light beam having returned from the P point contains the P-polarized light component, it is allowed by the first PBS 92 to pass through, while the light beam having returned from the Q point contains the S-polarized light component, it is reflected by the first PBS 92. Therefore, the light beams having returned from the P and Q points are superposed on each other in the first PBS 92 and incident upon the second lens 96. The superposed light beams are converged by the second lens 96 into an appropriate light beam and incident upon the BS 97. The BS 97 splits the incident light beam into two light components, of which one is incident upon the second PBS 98 and the other is incident upon the third quarter-wavelength plate 101. It should be noted that the second PBS 98 and third quarter-wavelength plate 101 are inclined 45 deg. in relation to the polarized direction of the incident light beam.
The second PBS 98 splits the incident light beam into two light beams: one containing S-polarized light component and the other containing P-polarized light component, and allows the light beam containing the S-polarized light component to pass through for incidence upon the first photoelectric transducer 99 while allowing the light beam containing the P-polarized light component to pass through for incidence upon the second photoelectric transducer 100. The first and second photoelectric transducers 99 and 100 produce interference signals of Acos (4Kx+δ) where K indicates 2π/Λ, x indicates a displacement, and δ indicates an initial phase. The first photoelectric transducer 99 produces an interference signal whose phase is shifted 180 deg. from that of an interference signal the second photoelectric transducer 100 produces.
Also, the third quarter-wavelength plate 101 produces, from the incident light beam, a circularly polarized light beam in which light beams containing P- and S-polarized light components, respectively, rotate in opposite directions and are superposed on each other to provide a plane-polarized light beam. This plane-polarized light beam is incident upon the third PBS 102. The third PBS 102 splits the incident plane-polarized light beam into a light beam containing S-polarized light component for incidence upon the third photoelectric transducer 103 and a light beam containing P-polarized light component for incidence upon the fourth photoelectric transducer 104. It should be noted that the polarized direction of the plane-polarized light beam incident upon the third PBS 102 rotates one turn when the diffraction gratings move Λ/2 in the direction of arrow x. Therefore, the third and fourth photoelectric transducers 103 and 104 can produce interference signals of Acos (4Kx+δ′) similarly to the first and second photoelectric transducers 99 and 100.
The third photoelectric transducer 103 produces a signal whose phase is 180 deg. different from that of a signal produced by the fourth photoelectric transducer 104. It should be noted that the third PBS 102 is disposed at an angle of 45 deg. with respect to the second PBS 98 and therefore the signals produced by the third and fourth photoelectric transducers 103 and 104 are 90 deg. different in phase from signals produced by the first and second photoelectric transducers 99 and 100.
The first differential amplifier 105 makes a differential amplification of the electric signals supplied from the first and second photoelectric transducers 99 and 100 to produce a signal in which a DC (direct current) component of the interference signal has been canceled, and supplies it to the incremental signal generator 107. Also, the second differential amplifier 106 makes a similar differential amplification of the electric signals supplied from the third and fourth photoelectric transducers 103 and 104 to provide a signal of which a DC component of the interference signal has been canceled, and supplies it to the incremental signal generator 107.
The displacement pickup 4 constructed as above is featured in that even a Y-directional move of the diffraction gratings will not cause any error of the position measurement because its optical systems are formed symmetrical with respect to a perpendicular line A in FIG. 1. Also, it is featured in that with equalization of the optical path for incidence upon the P point in length with that for incidence upon the Q point, it is hardly influenced by the wavelength of the light source.
Reference Cited Herein:
1. Japanese Published Examined Patent No. 35248 of 1990
The aforementioned displacement pickup 4 is used in an X-ray lithography or precision machining for production of an integrated circuit. For accurate measurement of a position or distance, a reference point or origin has to be set in addition to an incremental signal. The displacement pickup 4 includes the scale 108 to detect an incremental signal and the scale 108 to detect an origin signal. The scales 108 are provided in separate tracks, respectively, and a reading mechanism is provided for each of the scales 108. On this account, the measurement is affected by an Abbe error and an error caused by a temperature drift and thus the displacement pickup 4 cannot detect an origin with a high precision. Particularly, the conventional displacement pickup 4 cannot repeatedly measure any origin with a stable accuracy which is on the order of nanometers (nm).